An evaporator and/or a low-pressure vessel are an integral part of a refrigeration system. In a typical ammonia refrigeration system there is an evaporator that cools the process fluid at the expense of boiling the refrigerant that is at a lower saturation temperature and pressure, a compressor that compresses the boiled off refrigerant to an elevated pressure and temperature, a condenser that condenses the high pressure refrigerant to liquid phase at the expense of heating the cooling medium, and an expansion device that drops down the pressure of the condensed refrigerant back to the low side which then enters the evaporator to repeat the above cycle again. This cycle is called the reverse Rankine cycle.
Compressor is an integral and important part of this cycle. Compressor is also the major moving part in this cycle; therefore, it requires lubrication to overcome the friction between metal parts rubbing against each other. Certain quantity of this lubricant, which is generally mineral oil in an ammonia refrigeration system, escapes to other parts of the system. Generally the lubrication oil accumulates in the coldest part, i.e., the evaporator or a low-pressure vessel such as the recirculator vessel. Ammonia is evaporated in the evaporator but the oil does not boil off and remains as a liquid. There are three negative aspects of this oil migration and accumulation. Firstly, the compressor can eventually starve of oil and be damaged. Secondly, the financial loss due to constant replenishment and thirdly, large quantity of oil in the evaporator results in negative effect on the heat transfer characteristics of evaporator tubes or plates. Therefore, it is important that this oil be removed.
Several methods have been proposed and disclosed in previous patents such as U.S. Pat. No. 4,280,337 and U.S. Pat. No. 5,321,956 in which uses of various, pipes, valves and hold tanks is used. In the cited patents it is shown that the oil drainage is not a function of the amount of oil present in the evaporator or a vessel rather the oil is purged at a set time for a set period.